SPIROGYRA

also called Water-silk, Mermaid's Tresses, or Pond Scum (genus Spirogyra), any of a group of unbranched filamentous green algae, found in freshwater and usually free-floating. The slippery unbranched filament is composed of cylindrical cells containing one or more beautiful spiral green chloroplasts, from which the genus gets its name. The nucleus is suspended in the central vacuole by fine cytoplasmic filaments. Vegetative reproduction is by fragmentation of the filament. The sexual phenomenon of conjugation also occurs; cells of two strands lying side by side are joined by the outgrowth of conjugation tubes. Rarely, cells of the same strand join end to end, and the contents of one cell passes into and fuses with the contents of the other. The resulting fused cell (zygote) is surrounded by a thick wall and overwinters while the vegetative filaments die. On bright spring or fall days, masses of spirogyras float near the surface of streams and ponds, buoyed by oxygen bubbles released during photosynthesis. During the night, when photosynthesis decreases, the masses sink.